International application No. PCT/AU96/00637 (WO97/14206) describes a modular uninterruptible power supply (UPS) for connection, for example, between a mains power outlet and sensitive electric equipment such as a computer system. The disclosure of WO97/14206 is incorporated herein by reference. The modular UPS comprises a plurality of modules connected in parallel, each module being preferably “hot pluggable” in the housing of the UPS, whereby any module can be connected or disconnected from the housing without interrupting the flow of regulated electrical power from the UPS. Each module is provided with its own microcontroller that communicates via a control bus with control signal processing circuitry provided within the housing of the UPS. The control signal processing circuitry communicates with the microcontroller in each of the modules via a second control bus.
The control system processing circuitry allows a “majority rules” control system to be implemented whereby an alert control system output by each microcontroller is processed to determine the signal state of the majority of the microcontroller coupled in parallel, and the processed signal utilised to make a decision. For example, when a power fluctuation in the mains power supply is detected by the microcontroller of a module, that module outputs an alert control signal indicative of its detection, which is processed together with alert control signal from each other module connected in parallel. If the processed signal indicates that a majority of the modules have detected the power fluctuation, then the microcontrollers in all of the modules may act to supply power to the electrical apparatus from backup batteries rather than the mains power supply. In WO97/14206 the processing of the multiple alert control signals is effected by means of
an impedance network which “averages” the alert control signals from the modules, wherein a majority decision is made by the microcontrollers on the basis of whether the “averaged” output from the impedance netword is greater than or less than a threshold value.
Although the impedance network for effecting a majority decision as described in WO97/14206 works well in most situations, there is a drawback with this method in some instances. As the number of modules in the UPS increases the incremental effect brought by each module's alert control signal becomes progressively smaller. This can lead to a jittery situation near the majority point as the control signal processing circuit tries to determined whether a majority vote has occurred. For example, if there are 10 modules connected in the housing then each module contributes 10% to the signal averaging effect. When five modules are in agreement, the signals are nearly 50% and could easily be 49% or 51% due to tolerances of electronic components or noise.